Hydraulic brake systems in motor vehicles include a master cylinder to convert force exerted on the brake pedal into hydraulic pressure. The master cylinder provides the hydraulic pressure and volumetric displacement to operate the wheel brakes. The master cylinder includes a housing having a fluid reservoir and a cylinder bore which are connected by a port. A piston is slidable in the cylinder bore and carries seals between the piston wall and the bore to maintain pressure. There must be some structure to ensure that the piston does not block the port between the fluid reservoir and the cylinder bore during the filling of the system with brake fluid. Current master cylinder assemblies contain a spring which is located within the pressurizing chamber. The spring positions the piston to ensure the port is open when the brake system is first filled by an evacuation filling process in which a vacuum is created in the brake system and then brake fluid is introduced into the master cylinder reservoir and flows into the evacuated system. The springs also ensure the spacing between the piston and the closed end of the bore. In a dual circuit brake system, a spring ensures the spacing between the pistons during the operation of the master cylinder.
It would be desirable to eliminate the necessity for springs because the springs take up space in the pressurizing chambers even when compressed. The chamber must be big enough to allow the proper amount of fluid in addition to the space taken up by the spring. Eliminating the spring would allow the reduction of the size and weight of the master cylinder.